The function of myelin basis protein (BP) in vivo is unknown except for its role as a structural protein in myelin. It is susceptible to phosphorylation both in vitro and in vivo. Isolated BP contains only 2 phosphorylated residues, Thr-96 and Ser-153. Yet in vitro studies with 32P-labeled ATP show that other residues, Ser-12, Thr-35, and Ser-56, are readily labeled when myelin-bound BP is phosphorylated with myelin-bound protein kinase. The high degree of specificity of these reactions suggests that a study of phosphorylation in vivo might throw some light on the function of BP.